Conventionally, suspension polymerization of vinyl compounds (for example, vinyl chloride) has been conducted to obtain vinyl resins from the vinyl compounds. It has been known to use a partially saponified vinyl alcohol polymer (hereinafter, a vinyl alcohol polymer is sometimes abbreviated as “PVA”) as a dispersion stabilizer for suspension polymerization of vinyl compounds.
The requirements for a dispersion stabilizer for suspension polymerization of vinyl compounds include: (1) even if the amount of the dispersion stabilizer for suspension polymerization used is small, the resulting vinyl resin has high plasticizer absorptivity and is easy to work with; (2) it is easy to remove residual monomer components from the resulting vinyl resin; (3) the resulting vinyl resin contains fewer coarse particles; and (4) the dispersion stabilizer for suspension polymerization has good handleability.
Demand for dispersion stabilizers that are more effective in meeting these requirements (1) to (4) is increasing more and more. In particular, with respect to the above requirement (2) for easy removal of monomer components, for example, the restrictions on the amount of residual monomers in polyvinyl chloride used for medical and food applications are very severe. If it is difficult to remove residual vinyl chloride monomers in polyvinyl chloride particles in the drying step after polymerization, drying under high-temperature conditions or drying for a long time is needed to remove the residual monomers. In this regard, even for polyvinyl chloride for general applications, there is an increasing demand, for example, for energy cost reduction in such drying. With respect to the above requirement (4) for ease of handling, the use of organic solvents such as methanol is considered undesirable at present from the environmental standpoint, and there is a demand for a dispersion stabilizer for suspension polymerization in the form of a low-viscosity and high-concentration aqueous liquid.
Even if a traditionally-used common partially saponified PVA is used as a dispersion stabilizer for suspension polymerization to respond to these increasing demands for more effective stabilizers, it is rather difficult for such a common PVA to fully meet these enhanced requirements.
Patent Literatures 1 and 2 each disclose a method for making a partially saponified PVA more effective as a dispersion stabilizer for suspension polymerization and propose the use of a PVA having a terminal alkyl group for suspension polymerization of a vinyl compound. However, in this method, if the degree of saponification and degree of polymerization of the PVA are low, the PVA is insoluble in water and cannot meet the requirement (4), although it is effective to some extent in meeting the above requirements (1) and (2). The water solubility is improved by increasing the degree of saponification, but the requirements (1) and (2) cannot be met instead.
Thus, it can be said that no dispersion stabilizer for suspension polymerization of vinyl compounds that fully meet these enhanced requirements (1) to (4) has been found up to now. Nor can the PVA-based dispersion stabilizers for suspension polymerization disclosed in Patent Literatures 1 and 2 fully meet these requirements.